There is a general need for materials with low dielectric constants (low-k) in the integrated circuit manufacturing industry. Using low-k materials as the inter-metal and/or inter-layer dielectric between conductive interconnects reduces the delay in signal propagation due to capacitive effects. The lower the dielectric constant of the dielectric, the lower the capacitance of the dielectric and the lower the RC delay of the integrated circuit (IC).
Low k dielectrics are conventionally defined as those materials that have a dielectric constant lower than that of silicon dioxide—that is k<4. Typical methods of obtaining low-k materials include doping silicon dioxide with various functional groups containing carbon or fluorine. While fluorinated silicate glass (FSG) generally has k of 3.5-3.9, carbon-doping methods can further lower the k value to ˜1.9. Current efforts are focused on developing low-k dielectric materials with k values less than 2.5 for the most advanced technology needs. These ultra low-k (ULK) dielectrics can be obtained by incorporating air voids within a low-k dielectric matrix, creating a porous dielectric material.
Methods of fabricating porous dielectrics typically involve forming a film (sometimes referred to herein as a “precursor film”) containing two components: a porogen (typically an organic material such as a hydrocarbon) and a structure former or dielectric material (e.g., a silicon containing material). Once the precursor film is formed on the substrate, the porogen component can be removed, leaving a structurally intact porous dielectric matrix. Techniques for removing porogens from the precursor film include, for example, a thermal process in which the substrate is heated to a temperature sufficient for the breakdown and vaporization of the organic porogen. These thermal processes, however, have certain drawbacks. Substrate temperatures generally need to be high (i.e., greater than about 400 degrees Celsius) with exposure times typically on the order of hours. As is well known in the field, these conditions are unacceptable for backend-of-line applications as they can damage copper-containing devices.
Although the general approach of introducing voids into the dielectric will reduce the dielectric constant of the film, it will also reduce the density of the film and may sacrifice the mechanical strength and the thermo-mechanical properties of the film. Since dielectric films can be subjected to severe thermal and mechanical stresses in IC processes such as chemical mechanical polishing (CMP) and packaging, these porous films must have sufficient mechanical strength to withstand these processes. Therefore, improved methods of forming mechanically robust low-k porous films are needed.